Those skilled in the art are familiar with a problem that, in a point-to-point optical communication line (which comprises a number of network elements, such as optical fiber amplifiers OFA connected in series, and possibly, one or more optical add-drop multiplexers OADMs), one should pay attention to power equalization between multiple optical channels outputted by one network element to be sent to another one.
The amplifiers may work according to one of the following principles:
1. the principle of fixed output power (when the fixed output power is divided to a required number of the output channels—which leads to underpowering if the number of channels is max, and to a very dangerous overpowering where the number of output channels is low, say in a case of fiber cut before an OADM that adds few channels into an OFA.
2. the principle of fixed power dependent on the number of channels, wherein the amplifier output power is controlled with respect to the number of channels, so as to provide a fixed output power per channel. (for example, one channel—1 mW, 15 channels 15 mW, 40 or more channels—40 mW max). In the known technologies, the number of channels is usually stated (say, by an operator) from time to time.
3. the principle of fixed gain; according to that, the amplifier has a fixed gain so that each channel, whatever input power exists in it, is amplified with the same gain as other channels. To maintain the required output power, it is known to arrange a feedback loop measuring, the input and the output power and, based on the result, capable of regulating the output power of the amplifier. In the fixed gain scheme, in order to obtain equal and fixed output powers of the channels at the output of the amplifier, it is only possible to ensure that the input powers be equal and fixed. To maintain the condition, it is known to insert a VOA (Variable Optical Attenuator) into the input optic fiber, to adjust the input power of the optical channels which arrive to the amplifier.
The prior art does not comprise any idea of a dynamic span power equalization or control in a chain of network elements, comprising OFAs and OADMs. The problem stems from the facts that a) no dynamic monitoring of the chain parameters has been proposed, and b) elements of the chain may be different and thus behave differently, but no common concept of optical power control has been proposed for such a chain.